This invention relates to printing devices such as line printers. More particularly it relates to a magnetic interference preventing device in such a printing device, which is suitable for preventing the mutual magnetic interference of print magnets which are provided adjacent to one another to excite the respective print hammers.
In an impact type printing device comprising a number of print hammers arranged along a print line (position) and print magnets for exciting the print hammers, the print magnets are, in general, arranged extremely close to one another. This is to minimize space, and is shown in FIG. 1.
As is apparent from FIG. 1, when one of adjacent print magnets 2 is excited, any leakage flux thereof will be passed through the magnetic paths of the print magnets 2 adjacent thereto. Accordingly, adjacent print magnets which should not be excited are excited. In FIG. 1, reference numeral 1 designates the striking surfaces of the print hammers.
Printing a type in a column is carried out in accordance with a method in which the print magnet 2 is excited. It drives the print hammer by taking into account the flight time of the print hammer which is required for the print hammer to move from its rest position to the type striking position.
If, before a print hammer at a column is driven, a print magnet 2 adjacent thereto has been excited, then the energy supplied to the print hammer becomes different because the initial magnetic flux .phi..sub.O of the print magnet is not zero. As a result, the flight time is changed, printing the type at the correct position does not result and, in a worst case the type may not be printed.
In order to eliminate this difficulty, a method has been proposed in which a magnetic flux shielding plate is provided between the print magnets to prevent the magnetic interference of the print magnets 2.
However, the provision of the magnetic flux shielding plate makes the structure of the print hammer module intricate, and is one of the contributing causes which increase the manufacturing cost. Recently, there has been a strong demand for miniaturizing a printing device and accordingly its print hammers. If the size of the printing device is reduced, then it may be impossible to provide the magnetic flux shielding plates therein. Furthermore, even if the magnetic flux shielding plates could be provided in the printing device, the effect thereof would be insufficient. That is, it would be difficult to sufficiently prevent the magnetic interference of the print magnets.
The effect of the magnetic interference is most significant when both print magnets adjacent to, or on both sides of, a print magnet which is to be excited are being excited. More specifically, if, when the (N-2)-th column print magnet and the (N+2)-th column print magnet are being excited, the N-th column print magnet is also excited, then the effect of the magnetic interference is at a maximum.